1. Field of the Invention
The present invention relates to an aluminum alloy plate excelling in strength and corrosion resistance. More particularly, the present invention relates to an aluminum alloy plate excelling in strength and corrosion resistance which is suitably used for airplanes and vehicles, and to a method of manufacturing the aluminum alloy plate.
2. Description of Background Art
As an example of aluminum alloy structural plates, in particular, aluminum alloy plates for airplanes, a method of manufacturing a stringer material for airplanes has been proposed (Japanese Patents No. 1,337,646 to No. 1,337,649, No. 1,339,927, No. 1,405,136, and the like).
A specific example of the manufacturing method is as follows. An ingot of a JIS A7075 alloy is homogenized at about 450xc2x0 C. for 10-20 hours. The ingot is hot-rolled at 400-450xc2x0 C. to produce a plate material with a thickness of about 6 mm. The plate material is intermediate-annealed at about 410xc2x0 C. for one hour, and cold-rolled at 100xc2x0 C. or less to produce a cold-rolled plate with a thickness of 3-4 mm. The cold-rolled plate is subjected to a solution heat treatment by rapidly heating the plate to 320-500xc2x0 C., and aged at about 120xc2x0 C. for several to 24 hours to obtain a stringer material having a specific strength.
The aging step enables precipitation hardening to occur without causing the crystal grain size to change, whereby the resulting plate material has an average crystal grain size of 25 xcexcm or less and exhibits sufficient strength and formability for practical applications. However, even if the corrosion resistance, in particular, resistance to stress corrosion cracking, is judged as good in laboratory corrosion resistance evaluation, resistance to stress corrosion cracking is not necessarily satisfactory under a practical use environment. Therefore, further improvement of corrosion resistance has been demanded.
It is preferable to decrease the crystal grain size from the viewpoint of mechanical strength and formability of metal materials. However, a decrease in the crystal grain size may cause corrosion resistance to deteriorate. The present inventors have conducted experiments and examinations of the relation between a decrease in the crystal grain and resistance to stress corrosion cracking of a 7000 series aluminum alloy containing Zn and Mg. As a result, the present inventors have found that resistance to stress corrosion cracking is affected by the difference in crystal orientation (misorientation) between adjacent crystal grains.
As shown in FIG. 1, misorientation between adjacent crystal grains shows a degree of angular difference (misorientation xcex8) between a crystal grain 1 and a crystal grain 2 with respect to the common rotation axis. As a result of examination of the crystal grains after the solution heat treatment in the manufacture of stringer materials for airplanes, it was found that high angle boundaries with misorientations of 20xc2x0 or more were formed. In this case, grain boundary segregation of second phase compounds is increased during the succeeding aging. This causes the electrochemical characteristics to differ between the inside of the grain and the grain boundaries, thereby decreasing corrosion resistance.
The present invention has been achieved based on the above findings. The first object of the present invention is to solve conventional problems relating to an aluminum alloy structural plate and to provide an aluminum alloy structural plate excelling in strength and exhibiting improved corrosion resistance, in particular, resistance to stress corrosion cracking. Use of this aluminum alloy plate enables structures to be manufactured at reduced cost and improves reliability.
The second object of the present invention is to provide a method of manufacturing an aluminum alloy structural plate enabling the above aluminum alloy structural plate to be manufactured stably and securely.
The first object of the present invention is achieved by an aluminum alloy structural plate comprising 4.8-7% Zn, 1-3% Mg, 1-2.5% Cu, and 0.05-0.25% Zr, with the remaining portion consisting of Al and impurities, wherein the aluminum alloy structural plate has a structure containing 25% or more of crystal grain boundaries with misorientations of 3-10xc2x0 at the surface of the aluminum alloy plate. In this aluminum alloy structural plate, an average crystal grain size may be 10 xcexcm or less at the plate surface.
The second object of the present invention is achieved by a method of manufacturing an aluminum alloy structural plate comprising: homogenizing an ingot of an aluminum alloy having the above composition; hot rolling the ingot; repeatedly rolling the hot-rolled product at 400-150xc2x0 C. so that the degree of working is 70% or more to produce a plate material with a specific thickness; subjecting the plate material to a solution heat treatment at 450-490xc2x0 C. for five minutes or more; and cooling the resulting plate material at a cooling rate of 10xc2x0 C./second or more. The second object of the present invention is also achieved by a method of manufacturing an aluminum alloy structural plate comprising: homogenizing an ingot of an aluminum alloy having the above composition; hot rolling the ingot; repeatedly rolling the hot-rolled product at 400-150xc2x0 C. in a state in which a roll for hot rolling is heated at 40xc2x0 C. or more so that the degree of working is 70% or more to produce a plate material with a specific thickness; subjecting the plate material to a solution heat treatment at 450-500xc2x0 C. for five minutes or more; and cooling the resulting plate material at a cooling rate of 10xc2x0 C./second or more.